1. Field of the Invention
The invention relates to an image forming apparatus for detecting the size of recording sheet.
2. Related Background Art
Hitherto, for example, in a printer which cannot detect accurate size information of a recording sheet before it is fed, the distance to recording sheet detecting means is long and in order to realize a maximum throughput, the recording sheet detecting means has to execute the next paper feeding operation before detecting a rear edge of the recording sheet which was fed in advance. In a image forming apparatus with such a construction, paper size data and image data are received from an external computer and the image data is developed and, after completion of the development, the paper size data represents a universal setting and is sent to a printer engine and a printing start signal is transmitted. When receiving the printing start signal, the printer engine controls continuous paper feeding at a paper feeding timing which presumes a recording sheet of the maximum size which can be conveyed so as not to cause inconveniences such as image drop-out, jamming, and the like even in all of the paper sizes of specific lengths which can be conveyed by the image forming apparatus. The "universal setting" denotes that an arbitrary size which is equal to or less than the maximum paper size which can be conveyed and which is specified by the image forming apparatus is set.
However, there are the following problems in an image forming apparatus of a construction such that the size of papers stacked in a paper feeding unit cannot be detected before feeding the paper and a distance from the paper feeding unit to a recording sheet detecting means is long.
(1) In case of performing the universal setting, even if the paper size is small, since the continuous paper feeding operation is always performed at a paper feeding timing based on the maximum size, the throughput is not raised.
(2) In case of performing the universal setting, even if the paper size is small, since the continuous paper feeding operation is always executed at a paper feeding timing based on the maximum size, a time during which the apparatus operates is longer than it is needed, so that the life of apparatus or its reliability deteriorates.
(3) Each time the user tries to obtain the maximum throughput suitable for the paper size, the paper size has to be set from the external computer or the like.